1. Field of the Invention
The present invention relates to a front and rear wheel steering system for a motor vehicle, and more particularly to a front and rear wheel steering system which has a reduced electric power requirement while a motor vehicle is at rest and which has an increased response to an action to steer rear wheels of the motor vehicle when the motor vehicle starts to run from a stop.
2. Description of the Related Art
As well known in the art, front and rear wheel steering systems for steering rear wheels as well as front wheels of a motor vehicle determine a target steering angle for the rear wheels based on the driver's action to turn a steering wheel and the speed of the motor vehicle, and control a rear wheel steering mechanism to steer the rear wheels to the target steering angle. The rear wheel steering mechanism includes an actuator such as an electric motor or the like. The rear wheel steering mechanism also includes a neutral positioning spring for normally urging the rear wheels to a steering neutral position so that a fail-safe operation is ensured in the event of a failure of the front and rear wheel steering system.
The neutral positioning spring keeps the rear wheels urged to the steering neutral position irrespective of whether the motor vehicle is running or at rest. Therefore, even when the motor vehicle is at rest, once a target steering angle is established for the rear wheels, the electric motor is energized, i.e., consumes electric energy, to steer the rear wheels to the target steering angle against the bias of the neutral positioning spring. As a result, the front and rear wheel steering systems have a relatively high electric power requirement while the motor vehicle is at rest.
Japanese Laid-Open Patent Publications Nos. 1-82170 and 1-285463, for example, disclose front and rear wheel steering systems that are designed for a low electric power requirement while the motor vehicle is not running.
More specifically, the front and rear wheel steering system disclosed in Japanese Laid-Open Patent Publication No. 1-182170 detects when the motor vehicle is parked, and cuts off the supply of electric energy to the electric motor of the rear wheel steering mechanism, thereby reducing the electric power consumption, when the motor vehicle is parked.
However, when the motor vehicle is parked, since the electric motor is de-energized, the rear wheels are forced to return to the steering neutral position under the resiliency of the neutral positioning spring. At the time the motor vehicle starts to run from the parked condition, the front and rear wheel steering system is required to steer the rear wheels again to a target steering angle. Consequently, a certain delay cannot be avoided in steering the rear wheels initially when the motor vehicle begins to move. In addition, when the motor vehicle repeatedly stops and runs frequently, the front and rear wheel steering system consumes a large amount of electric energy because it must steer the rear wheels to a target steering angle each time the motor vehicle starts to run from a stop.